The present invention relates to an engine cooling apparatus, and more particularly to an engine cooling apparatus having at least two engine cooling systems around a combustion chamber of an engine.
If the temperature of a combustion chamber of an engine were too high, knocking would frequently be caused, and the intake charging efficiency would be lowered, resulting in a reduction in the engine output power. Since the valve operating system in a cylinder head produces frictional heat, it is generally desirable that the upper portion of the combustion chamber be cooled to a relatively low temperature. On the other hand, the side and lower portion of the combustion chamber nera the cylinder block should preferably be cooled to a relatively high temperature. More specifically, the mutually sliding surfaces of the combustion chamber and the piston and the mutually sliding surfaces of the bearings of the crankshaft produce frictional heat due to their fitting engagement and dependent on how oil films are formed on these sliding surfaces. It is better to keep the sliding surfaces at a relatively high temperature for reducing the frictional resistance and hence minimizing any reduction in the engine output power.
Japanese Laid-Open Patent Publication No. 51-124749, for example, shows an engine cooling arrangement in which the cylinder head is cooled by cooling water of a relatively low temperature and the cylinder block is cooled by cooling water of a relatively high temperature. Inasmuch as the cooling water starts to boil and produce air bubbles normally at about 100.degree. C., there is a limitation on relatively high temperatures up to which the side portions of the cylinder block are kept by the cooling water. Improvements remain to be made in this respect.